Oftentimes in a player’s biography you read that so-and-so grew up as a fan of the Lakers or Celtics or Yankees, or whatever while a kid/adolescent, but it seems to have zero impact on their professional careers. They play(ed) against their favorite teams just as hard as against any other opponents - in fact, perhaps even harder - and didn’t seem to have any particular desire to play for their favorite team (except perhaps Matt Cassell, I heard he was a big Cowboys fan as an adolescent but even then that seems to have played only the tiniest of factors in luring him to Dallas later as a free agent for one season.) Is it that once they turn pro, there are only “My team vs. all other teams in the league” and they instantly drop any vestige of previous fandom?
You sometimes see players returning to childhood teams - occasionally a hometown discount.
Ah neat, thanks. I would also be curious if high school recruits are likelier to sign with a college team of their fandom - i.e., if a teen grew up a Notre Dame fan, is therefore likelier to sign with Notre Dame. I would guess that teens are more impressionable and prone to making emotion-based decisions than adult pros in their 20s or 30s.
Depends entirely on the talent level of the recruit. A 5 star recruit that’s almost certain to go pro is going to look more at the schools that will help them develop the most and give them the biggest audience for exhibition. The 3 star that only got offers from maybe a dozen schools might look more closely at the hometown school.
I hear about it all the time in football. Earl Thomas infamously and openly wanted to go to the Cowboys because he has always been a fan (he’s from Orange, Texas and went to UT Austin in college). Marshawn Lynch, who is from Oakland and grew up a Raider fan, and was never quiet about being a fan. He came out of a one year retirement just because he wanted to play for them (and he’s been a Raider since last year).
So not only do players not hide their fandom, sometimes they try to leverage success on a different team to be able to play for the team they grew up rooting for.
But adult pros who are starting out don’t get to choose at all (well, once a decade or so a top college quarterback can insist on avoiding a particular team; never heard of it for anyone else). And even later in their career the number of adult pros who can pick and choose where they go is basically in the single digits. The vast majority of players will either never have a choice or at best will pick from two or three competing offers.
So their only real choice is play for the team that drafts them/trades for them, or don’t be a professional athlete. If that’s the choice, most of them can get over playing for a childhood fandom enemy. And remember, you don’t get to be a pro athlete without being competitive, so once they’re on a team, they’re not going to pull back against a childhood favorite team.
It was a pretty big deal, in the NHL, when John Tavares signed with his hometown team, the Leafs.
I’d expect the opposite. The athlete would more likely be excited for the opportunity to play on the same field as his childhood team, and prove himself in front of and against them. He definitely wouldn’t want to embarrass himself.
Also, if he’s playing in that team’s stadium he’s playing near where he grew up and it’s more likely that family and friends from his home town will be there to watch, which is even greater incentive to perform well.
There was a time in the 1990’s and early 2000’s when the Liverpool team was full of local boys, all of whom were Everton supporters. Did not seem to stop them from winning lots of trophies.
Harry Kane of Tottenham grew up an Arsenal fan.
I also recall seeing a YouTube video about a Gridiron player who was a big fan of his local team and they passed on him. He turned out be a multiple player of the year type. Not a follower of the sport, but I think it was Manning or Tom Brady.
Everyone passed on Brady, some 5 times and some 6. New England took him in the 6th round with the 199th overall pick. Manning was drafted 1st overall.
I would guess between the two you’d be talking about Tom Brady. Peyton Manning was the first overall pick in the 1998 draft so nobody “passed” on him. Tom Brady was the 199th pick taken in the sixth round in the 2000 draft, so every team passed on him multiple times. 
Brady wasn’t a very hot prospect coming out of college and didn’t become a starter until his second year when Drew Bledsoe, the starter for New England, was out with an injury. It’s no surprise that he wasn’t highly-regarded; even the team that drafted him waited to start him until they had to. Of course once he became a starter he quickly proved himself because he led the Patriots to a Super Bowl championship win that year and the rest is history.
Brady grew up in the general San Francisco area and watched multiple 49ers games at Candlestick Park as a kid. He idolized Joe Montana and was at the game when the 49ers famously beat the Cowboys in the 1981 AFC Championship. I would not be shocked to hear that he was disappointed that he didn’t get drafted by Sam Francisco. I’m sure by now that organization regrets it too… 
ETA: Partially ninja’d
And, five years later, the 49ers had the first overall pick in the 2005 draft, and still needed a quarterback. They had an opportunity to draft another local kid as QB, Aaron Rodgers (whose pedigree coming out of Cal was considerably stronger than Brady’s had been), and many analysts assumed that the Niners would go with Rodgers. Instead, they took Alex Smith, and Rodgers slid all the way to pick #24, where the Packers were thrilled to land him.
Smith eventually became a reasonably good QB for the Niners, though he eventually lost his job to Colin Kaepernick. Rodgers, meanwhile, wound up on the short list of best QBs of his generation.
So, seems the moral of the story is that SF suck at drafting quarter backs!
How have they done since Brady and this Rodgers fellow debuted, compared to those players record with their teams?
Are the teams they play for major rivals?
Oh, I just remembered another one. John Terry supported Man U growing up, but played for Chelsea and misses what would have been the winning penalty in the 2008 Champions League Final, versus Man U.
Since you’re not familiar with the NFL, I’m not sure how tongue in cheek you’re being about the 49ers and QBs. They’ve had a couple very good QBs, Joe Montana is in the running for best ever.
They’ve been pretty bad in recent years though. Neither New England nor Green Bay are big rivals of the 49ers. They basically never play New England and rarely play Green Bay.
Barry Bonds went to play in SF because his father Bobby played there.
Indiana University basketball product Victor Oladipo now plays for the Indiana Pacers (he was traded there). Last year was his first in a Pacers uniform, and he seems to take great delight in being a Pacer. He also put up spectacular numbers, and was named to the All-Star team for the first time.
Every now and then after hitting a big shot, he’ll make a motion with his hands and say “This is my city!” It’s refreshing for a Pacers fan to see someone so happy to be there.